AN1902 Freescale Semiconductor / Motorola, AN1902 Datasheet - Page 9

• Manufacturer Part Number: AN1902
• Description: Quad Flat Pack No-Lead (QFN)
• Manufacturer: Freescale Semiconductor / Motorola

MOTOROLA
4.1.2 Ro0bust Design Guidelines
4.1.2.1 Perimeter Pad Design Guidelines
The two main PCB perimeter pad design factors are the pad (terminal) length and the
spacing between pads (depicted as edge-to-edge spacing in
vides a summary of package pad width and spacing between package pads from the
case outline drawings.
The 0.20mm package pad edge-to-edge spacing (for maximum pad width limit) can be
challenging in the PCB layout. The PCB pad width can be designed smaller than the
maximum possible package pad width dimension to allow for greater assembly pro-
cess window. More common in the industry, would be to use a 1:1 ratio between the
nominal package pad width and the PCB pad width. In this case, the 0.50mm pitch
QFN package has a package pad edge-to-edge spacing of 0.26mm, which is more
easily manufactured.
The concern shifts from shorting between neighboring perimeter pads to pad-to-
exposed pad (die attach paddle) shorting. The larger the spacing between the PCB
perimeter pads and exposed pad, the less likely for solder shorts due to tolerance
stacking.
For assembly, the PCB EP should be smaller than the nominal package QFN so that
at least 0.250mm of space between input/output (I/O) leads and EP edge exists.
Pad Width Limits
Minimum
Nominal (Center)
Maximum
Freescale Semiconductor, Inc.
Case Outline
For More Information On This Product,
Table 3 Case Outline Dimensions for QFN Package Pad Width
Table 4
Quad Flat Pack No-Lead (QFN)
Go to: www.freescale.com
provides pad-to-exposed pad spacing for various QFN packages.
0.18mm
0.24mm
0.30mm
Pitch QFN
0.50mm
Figure 11 PCB Pad Dimensions
Pad Spacing
0.32mm
0.26mm
0.20mm
0.23mm
0.30mm
0.37mm
Pitch QFN
0.65mm
Figure
11).
0.42mm
0.35mm
0.28mm
Pad Spacing
Table 3
AN1902/D
pro-
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